Special equipment: a waffle maker
Preheat a waffle maker to the regular setting according to the manufacturer's instructions.
Butter each piece of bread on 1 side. Flip 1 piece and spread the non-buttered side with pesto. Top with the sliced mozzarella and tomatoes and lay the second piece of bread on top buttered-side up. Place on the waffle maker, close the lid and cook until melted and golden.